Ink jet printing systems are widely used in many different fields to produce printed images on a variety of substrates. Conventional color ink jet printers utilize basic process ink color sets, usually comprising from four to eight process colors, to form the color variations required to print the desired image. One of the well-known process ink color sets comprises four basic colors: cyan, yellow, magenta, and black, and is referred to as the CMYK color set or the CMYK process color method. Generally, the process colors in a process color set are typically used to produce a range of printed colors by combining various amounts of each of the process colors. Typically, in an ink jet printer, the CMYK colors are not actually blended together to form the desired color, rather very small drops of ink of different colors are deposited next to each other on the page. From a slight distance, the human eye tends to blend the individual ink drops together to form an area of the “mixed” color. The range of possible colors that may be printed by a printing process is referred to as the “gamut” of the process.
Using these techniques, color ink jet printers are generally able to reproduce a sufficient number of colors. In an effort to expand the gamut of the CMYK color set, other process inks have been added, thus creating other process color sets/methods, e.g., CMYKRGB and CMYKOG. Unfortunately, the gamuts of most of these color sets are much smaller than the total range of colors that can be seen. The gamuts of process color printers are smaller than the gamuts produced by traditional printing presses, such as offset, flexographic, gravure and screen printing presses. For example, flesh tone colors are not easily created using the CMYK color method, nor are fluorescent and metallic colors.
Also, many specialized colors cannot be easily formed via conventional process ink color sets. Some specialized colors are referred to as spot colors (also house colors or brand colors). These spot colors are customer-specific multi-pigmented colors often associated with a specific company, product, and/or brand name. Spot colors are generally formed at the molecular level and are made up by bending a variety of tints, hues and shades of colors. Examples include the precise hues of red and yellow used by Kodak, IBM Blue or Coca-Cola Red. Conventional ink color sets are usually unable to reproduce spot color vividness. This inability is perhaps due to the fact that in conventional process printing, every color is made up of overlapping process ink dots, as opposed to spot color printing, which involves application of solid areas of the pre-mixed spot colors. When CMYK inkjet printers try to achieve some of these specialized colors, processing problems ensue. In addition, there are often slight variations in the CMYK ink lots, so even if a specialized color of ink is achievable at one time, it may not be achievable at a later time, if ink of a different lot is used. Thus, the color gamuts provided by conventional ink color sets leaves much room for improvement.
In addition to the color gamut deficiency, conventional ink color sets/processes yield printed images that are insufficient for various applications. For example, problems are known to exist with the durability of process ink images. Specifically, for applications relating to large format signage, e.g., outdoor signage such as traffic signs, printed images produced from conventional ink color sets have been known to fade over time, to decrease in reflectivity, to vary between nighttime and daytime color, and/or to demonstrate other decreases in visual quality, e.g., sharpness of color and color contrast. These parameters are particularly germane to traffic signage because of the strict regulations relating thereto. Also, conventional images are known to have problems with consistency. That is to say these images suffer from defects such as banding and other types of color variations. In some cases, these types of defects are difficult to initially recognize due to the composition of the process color inks that are employed.
Further, conventional printers may include several process color channels and a small number of spot color channels (as compared to the number of process color channels). If an additional spot color is needed (as is often the case) one or more of the spot color channels must be flushed with cleaning solvents (often referred to as the longstore process). The longstore process is time consuming and generates much waste of inks and cleaning solvents. Further, the increased use of cleaning solvents has a significant negative effect on printer life.
US Patent Application Publication No. 2012/0090488 discloses that spot colors reproduce high value brand colors to supplement conventional process colors for printed images and packaging. This publication also discloses that a printing press can utilize hi-fidelity process inks (with at least three unique colors and black) and spot colors for printing solid brand colors and for printing over other colors to produce wide-gamut, half-toned color reproductions. The publication further discloses a method for printing logos on a substrate including (i) printing the spot colors using spot inks, and (u) [sic] printing the remainder of the image using a substituted original process ink set. Such method can alternatively include (in) [sic] reading the spectral characteristic of the inks, (iv) determining if any color in the image is identified by a color management method from the default process ink set palette, (v) processing the out-of-palette-colors using spot colors and the standard process ink set, and (vi) applying a tone scale value increase (TVI) to the process color sets. This publication, however, focuses exclusively on problems related to expansion of the gamut of conventional ink color sets and makes no mention of durability, consistency, control, or printer life, etc.
Other references also relate to expansion of color gamuts of conventional ink color sets and processes, many of which add process inks to the existing color set and some of which used process colors in attempts to simulate spot colors. Some references include U.S. Pat. Nos. 6,717,699; 7,004,562; 7,032,517; 8,955,940; and US Patent Application Publication Nos. 2015/0158317 and 2015/0339552. These references, however, make no mention of incorporation of a spot color into an extended gamut process set.
Although the references may teach the use of revised ink color sets to expand color gamuts, a need still exists for improved ink color sets and processes relating thereto that provide the beneficial combination of improvements in printed image durability, printed image consistency, control, and printer lifetime (longstore process reduction/elimination) in addition to significant additional gamut expansion.